1. Field of the Invention
The present invention relates to an anti-skid control apparatus for a vehicle, and particularly to an anti-skid control apparatus from which a pump, and a motor for driving said pump, used for pumping brake fluid stored in a reservoir to a master cylinder, have been omitted.
2. Description of the Related Art
As progress has been made reducing the cost of anti-skid control apparatuses, a pumpless anti-skid control apparatus has been proposed. Based on a circulating type fluid pressure control device, this pumpless anti-skid control apparatus eliminates the pump used for pumping brake fluid expelled by a wheel cylinder and stored to a reservoir into the discharge chamber of the master cylinder, and eliminates the motor for driving said pump. A problem with this type of pumpless system, however, is that anti-skid control can only be continued until the reservoir becomes filled with brake fluid. To extend for as long as possible the time that anti-skid control can be sustained, it was necessary to reduce the pressure reduction while also extending as long as possible the generation cycle of wheel speed skidding during anti-skid control.
It was therefore proposed in Japanese Patent Laid-Open Publication (unexamined) No. 63-301155 to add a cut-off valve to a pumpless circulating-type anti-skid system, provide a master cylinder back pressure valve between the inlet solenoid and cut-off valve, and minimize repressurization by supplying pressure lower than the master cylinder pressure to avoid wheel locking.
Japanese Patent Laid-Open Publication (unexamined) No. 6-298060 discloses a pumpless system for stopping a vacuum booster and using reduced pressure; when increasing the fluid pressure, this system supplies brake fluid from the master cylinder, but when pressure reducing continues for longer than a specific time, booster operation is stopped, or the booster is driven in the opposite direction, that is, in the direction causing the brake pedal to return, moving the piston of the master cylinder in the pressure reducing direction to lower the master cylinder pressure.
As described above, Japanese Patent Laid-Open Publication (unexamined) No. 63-301155 and Japanese Patent Laid-Open Publication (unexamined) No. 6-298060 propose suppressing the master cylinder pressure during anti-skid control. A pumpless anti-skid system must also enable anti-skid control when the skid cycle is long, however, and this requires that the wheel cylinders be repressurized to achieve a constant repressurization amount irrespective of the pedal pressure applied to the brake pedal by the driver. Generally speaking, when the pedal pressure applied by the driver increases, the master cylinder pressure increases, and the increase in the wheel cylinder pressure achieved for an increase-pressure command time increases proportionally to the master cylinder pressure.
As described above, the amount of repressurization during anti-skid control varies according to the variation in brake pedal pressure applied by the driver, and the master cylinder pressure varies according to the control conditions. As a result, an appropriate increase in pressure is not achieved, and the length of time during which anti-skid control can be sustained becomes shorter. Therefore, to achieve a specific increase in pressure relative to a particular wheel cylinder pressure irrespective of the brake pedal pressure applied by the driver, the pressurization time must be variably set. This cannot be achieved without detecting the master cylinder pressure using some kind of sensor, and this use of sensors increases cost.
Furthermore, when the friction coefficient .mu. of the road surface changes suddenly from a high .mu. to a low .mu., a large volume of brake fluid flows into the reservoir, and the friction coefficient .mu. of the road surface then again changes suddenly to a high .mu., there is the problem that the tolerance for depressing the brake pedal becomes smaller, and deceleration of the vehicle cannot be recovered.